I've seen examples of actual production systems where there is just a general proliferation of slow practices that make the system slow without any one function standing out yes. In C# such practices could include using interfaces (for dependency injection) and linq (for readability), and reflection for instance. All of these have fast alternatives with varying levels of downsides. The downsides aren't as big when you are practiced at them.

Also, when profiling production code, it can be hard to FIND the slow function since the optimizer may inline things.

There is perhaps value in understanding your language and ecosystem very well, so that you have decent intuition for what is fast, and making default-fast choices even if their is a small readability cost. That cost may be made up for by not having to crunch on performance later. As well, many performance choices make code simpler, after all the goal is to do less. and less is less.